Mercury Mass Balance in the Adirondack Mountains:
Stemflow and Soil Concentrations
Lamya Badr1 and Victoria Boyd2
Civil and Environmental Engineering Department
Introduction
Mercury is naturally found in soils, forests, lakes, and open oceans (Choi); however
anthropogenic sources are increasing the amount of mercury found in the environment. Such
anthropogenic sources include coal-fired power plants, waste incinerators, electric utilities, industrial
manufacturing, wastewater treatment plants, and improper disposal of consumer products (Driscoll). Of
these coal-fired power plants is the largest category (Choi). As mercury is emitted into the atmosphere,
either wet deposition or dry deposition moves it from the atmosphere to the land surface. Wet deposition
refers to deposition that takes place with rain or snow while dry deposition occurs with particulate
mercury and reactive gaseous mercury in the absence of precipitation (Driscoll). Mercury is emitted into
the atmosphere in three different forms: elemental mercury (Hg0), gaseous reactive mercury (Hg2+ or
RGM), and particulate mercury (PHg). These different forms of mercury each have different atmospheric
residence times. While gaseous and particulate mercury both have residence times of about 0.5 to 3 days,
elemental mercury has a residence time of 0.5 to 2 years. Therefore, elemental mercury can travel the
farthest indicating that its emission is a global, not a local, problem. The unnatural increase of Hg
concentrations in the air and soil are problematic because mercury is known to be toxic to the human
body as well as the environment. Methylmercury, when deposited into a body of water, can easily build
up in the aquatic food chain. This can result in reduced fertility, slower growth and development, and
mortality. Mercury’s effects on humans are similar; exposure to methylmercury in the womb directly
affects the neurological development in the child impacting the memory, thinking, attention, and language
skills (Mercury).
The purpose of this research was to improve the understanding of how Hg behaves in the
Adirondack Mountains of New York State. Although a preliminary mass balance of Hg has been
1
2
Class of 2009, Civil Engineering, Virginia Tech, REU, Holsen
Class of 2010, Environmental Engineering, Clarkson University, REU, Holsen
constructed for this region there are several unknowns including how much Hg is in the soil and how
much Hg is entering the system associated with stemflow (water that runs down tree branches and trunks
to the soil system. Stemflow makes up only about 2% of gross annual precipitation but the Hg
concentration is expected to be high due to dry deposited Hg which has accumulated in the canopy and is
rinsed off in the stemflow (Kolka). Hg concentrations in soil are expected to vary depending on the forest
canopy. Due to the higher organic content of deciduous soils it is expected that Hg concentration will be
higher in deciduous areas than coniferous areas. Soil concentrations are also expected to change with
depth and distance from tree trunks.
Methods
Stemflow
Water samples were taken at two different sites in Huntington Wildlife Forest of Newcomb, New
York (43.97°N, -74.22°W). A beech tree with a diameter of 90cm and a white pine tree with a 45cm
diameter were chosen for testing. The stemflow collector was made from materials that are chemically
inert to insure no Hg was lost or gained during sample collection. Stemflow was collected using Teflon
tubing that spiraled down the trunk of the tree. The tubing was cut down the center to allow for water to
flow down the spiral and was secured to the tree using 100% silicone caulk and nails. The nails were
completely covered with the caulk so that no water touched the metal. The collection container was a 20
liter glass container. Tests done to determine the impact of the silicone caulk on the samples showed that
there was little impact on Hg concentration. Collectors were also made to collect throughfall and rainfall
in order to compare the volumes and concentrations with stemflow. Samples were analyzed in a Tekran
2600 and EPA Method 1631.
Soil Inventory
To determine if Hg concentration varies with increased distance from tree trunks due to inputs
from stemflow soil samples were taken in radial patterns away from tree trunks near both deciduous and
coniferous trees. A sugar maple tree with a diameter of 180cm and a white pine tree with a 50cm
diameter were chosen for testing. Samples were taken using a sterile spatula every centimeter for the first
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Class of 2009, Civil Engineering, Virginia Tech, REU, Holsen
Class of 2010, Environmental Engineering, Clarkson University, REU, Holsen
meter and every 10 centimeters for the next half meter in order to determine if concentration changes with
increased distance from the trunk. Samples were taken in southwest and northeast directions because that
has been shown to be the predominate path taken by runoff in the Adirondack area. Soil samples were
also taken every centimeter for 30 centimeters into the ground to find the change in concentration with
depth. All samples were analyzed using a Milestone DMA-80 which uses EPA Method 7473, thermal
decomposition, amalgamation/atomic absorption spectrophotometry method (TDA/AAS) to determine the
total Hg concentration in the soil. The moisture content of each sample was found by drying the soil in a
tin pan for 16 hours at 105°C. The moisture content was used to determine the dry weight of all samples.
Results and Discussion
Stemflow
Stemflow samples are expected to have a higher Hg concentration than the throughfall and wet
deposition samples. The deciduous tree was larger than the conifer tree and, therefore, more Hg is
expected to be washed down the trunk resulting in a higher concentration.
Soil Inventory
The conifer tree analysis had unexpected results. Neither the northeast or southwest sides of the
tree showed an obvious trend. The majority of the samples from the northeast side fell between 70 and
110ng/g while the southwest side ranged from 80 to 120ng/g (Figure 1 and 2).
Figure 1: Hg concentration in soil vs. distance from conifer tree in the southwest direction.
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2
Class of 2009, Civil Engineering, Virginia Tech, REU, Holsen
Class of 2010, Environmental Engineering, Clarkson University, REU, Holsen
Figure 2: Hg concentration in soil vs. distance from conifer tree in the northeast direction.
The percent moisture for the northeast side of the tree ranged mainly from 20% to 40% but the
variance was high. The southeast side of the tree was less varied with only 3 samples falling outside of
the 30% to 50% range. The percent moisture will be used to find the corrected concentration based on
dry weight. Due to the consistency of the percent moisture it is expected that the overall curve of the
graph will illustrate the same pattern.
Conclusion
From the data analyzed so far the soil concentrations did not decrease with distance from the tree
as expected. Instead, the samples seem to fall within a certain range with a few outliers. The samples for
this research were only collected from two different trees. More samples must be taken in different areas
before any conclusions can be made about mercury concentrations.
Works Cited
Choi, Hyun-Deok, 2007. Mercury inputs, outputs, cycling, and ambient concentration under the forest
canopy if the Adirondacks of New York. Clarkson University Civil and Environmental
Engineering Dept, 4.
Driscoll, Charles T.; Han, Young-Ji; Chen, Celia Y.; Evers, David C.; Lamber, Kathleen Fallon; Holsen,
Thomas M.; Kamman, Neil C.; Munson, Ronald K. Mercury Contamination in Forest and
Freshwater Ecosystems in the Northeastern United States. Bioscience. 2007, Vol.57 No.1, 17-28.
Kokla, R.K.; Nater, E.A.; Grigal, D.F.; Verry, E.S. Atmospheric Inputs of Mercury and Organic Carbon
Into a Forested Upland/Bog Watershed. Water, Air, and Soil Pollution. 1999, 113, 273-294.
Mercury: Human Exposure. July 14 2008. U.S.E.P.A. June 6 2008.
http://www.epa.gov/mercury/exposure.html>.
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Class of 2009, Civil Engineering, Virginia Tech, REU, Holsen
Class of 2010, Environmental Engineering, Clarkson University, REU, Holsen